1. Field of the Invention
The present invention concerns a method for changing the characteristic curve of a regulation valve, the valve consisting of a housing provided with a through flow duct and of a closing member, the closing member is fitted inside the valve housing in the flow duct, and is shaped as a body of rotation or as a part of a body of rotation, and is rotated by means of a turning shaft extending to outside the housing, the shape of the forward edge of the closing face of the closing member or of the flow opening in the closing member differing from the shape of the rearward edge, and the characteristic curve being determined by the shape of the edge of the closing face of the closing member at whose side the flow opening starts being formed when the valve is rotated from the closed position to the opened position. The invention also concerns a regulation valve consisting of a housing provided with a flow through duct, a closing member which is placed in the flow duct inside the housing and which is shaped as a body of rotation or as a part of a body of rotation, and a turning shaft extending from the closing member to outside the housing. The shape of the forward edge of the closing face of the closing member or of the flow duct in the closing member differs from the shape of the rearward edge.
2. Overview of Related Art
The invention is particularly suitable for rotation-regulation valves, such as globe, segment, or plug valves. It is a feature common of these valves that, when the closing member is rotated from the closed position to the opened position, an opening starts being formed beginning from one edge of the closing face of the closing member. Such a valve comprises a housing provided with a flow duct, a closing member fitted in the valve rotatably, and a turning shaft employed for rotating the closing member and passed to outside the housing by means of packing members. A separate seal has been fitted into the housing, or a sealing face has been machined into the housing, which said seal or sealing face may be in contact with the sealing face of rotation on the closing member. Alternatively, there may be a gap between the seal and the closing member, in which case, in the closed position, the flow through the gap is small compared with the flow when the valve is fully open.
In regulation use, performance of the valve can be illustrated by an internal characteristic curve of the valve. The curve represents the flow quantity of a non-compressible medium in relation to the opening angle or the stroke of the valve under standardized conditions while the pressure remains unchanged. The curve is presented by means of the capacity coefficient Cv (Kv) of the valve, by whose means it is possible to calculate the quantity of medium flowing through the valve when the difference in pressure across the valve and the density of the medium are known.
The characteristic curve illustrates the effect of a control signal on the flow quantity of the valve. The shape of the curve is a characteristic feature of the valve which affects the control. Two typical shapes of a characteristic curve are a linear model, and an equi-percentage model. In the linear model, the opening angle and the flow quantity in the valve are at an invariable ratio. On a linear scale, the graph is a straight line. In the equi-percentage model, a standard change in the opening angle or in the signal causes a change in flow that is equal to a preceding change expressed as a percentage. The graph of the flow/opening angle is an exponential curve.
The choice of the valve on the basis of the characteristic curve depends on the object of regulation and on the circumstances. The controllability is also affected by the external control circuit (pipe system). By means of the process controller and the feedback of the position control of the valve, it is still possible to change the model of behavior, but in view of general usability, it is advantageous if the model can be changed readily in the valve itself.
The capacity factor of a valve determines the suitability of the valve for the planned operation. If the requirement of capacity is higher than the capacity of the valve under the conditions concerned, a required quantity does not flow through the valve. On the other hand, if the capacity is excessivelY high, this means that the operation of the valve takes place within a little area of regulation near the closed position. In such a case the geometric errors and plays in the valve, in the actuator, and in the controller are high in relation to the opening angle or to the stroke, and the circuit does not become stable. The problem is so difficult that attempts have been made to solve it by means of actuators of special constructions, wherein the actuator/controller is tuned to operate in its entire range of operation even if the stroke required by the valve were only a part of the range of operation.
In regulation operation, it is known in prior art to exchange the regulation member of the valve in accordance with the requirements imposed by the conditions of operation. This mode is common in particular in regulation valves of seat type. Exemplifying embodiments are described in the U.S. Pat. No. 3,821,968 and in the German publication No. DE 2,359,717. The replacement can take place only by disassembling the valve.
A construction is also known wherein there is a separately turnable regulation disk in the flow duct in a globe valve/plug cock, U.S. Pat. No. 2,809,662, or wherein there is an exchangeable throttle plate, U.S. Pat. No. 3,610,286.
A drawback of these constructions is that they are complicated and, consequently, expensive. When the change requires replacement of components, the pressure in the pipe system must be lowered to zero. Moreover, in a vertical pipe, the pipe must be emptied for the time of the replacement.
Such globe valves are also known in prior art wherein the shape of the inlet edge of the closing face of the ball differs from the shape of the outlet edge of the closing face. Such valves are described, e.g., in the GB Patents No. 1,151,661 and No. 1,395,038 as well as in the U.S. Pat. No. 4,037,818. These prior-art valves, however, always have the same opening direction and the same closing direction.